1. Field of the Invention
The present invention relates to a bus network, and more particularly, to a bus network to which stations can be accessed at high speed and efficiently when one of the stations transmits information to the other stations.
2. Description of the Background Art
It is expected that a network utilizing an optical fiber for a transmission line will be widely employed in the future because of the advantages that it can transmit a large amount of information utilizing the wide band characteristics of the optical fiber, and the optical fiber is not easily affected by noise.
In a transmission system utilizing the optical fiber, a system in which an exchange and a repeater positioned in the above-mentioned network convert a light signal, sent from a transmission terminal, into an electrical signal, then reconvert the electrical signal into the light signal, and send the light signal to a receiving terminal for receiving information, has been widely utilized. An example follows.
"First Background Art"
FIG. 13 is a block diagram showing the construction of a network described in the first document "F. E. Ross, "An Overview of FDDI: the Fiber Distributed Data Interface," IEEE Journal on Selected Areas in Communications Vol. 7 No. 7 pp.1043-1051, September 1989". In FIG. 13, each of the terminals comprises an optical/electrical converting device (illustrated as O/E) and an electrical/optical converting device (illustrated as E/O) in its inside, and constitute a ring network using an optical fiber. Each of the terminals converts a light signal sent from a previous terminal into an electrical signal, then converts the electrical signal into the light signal, and sends the light signal to a subsequent terminal.
In the transmission system, it has been pointed out that conversion from the light signal to the electrical signal (or conversion from the electrical signal to the light signal) is frequently performed within the network, whereby it is difficult to increase the number of terminals connected to the network and to increase the transmission speed of information.
In recent years, an optical network capable of sending a light signal from a transmission terminal to a receiving terminal without converting the light signal into an electrical signal has been examined. Further, a star network has the advantage that the transmission loss is smaller, as compared with a bus network. Therefore, many star networks have been conventionally studied and developed. A description of examples of the star optical network follows.
"Second Background Art"
FIG. 14 is a block diagram showing the construction of an optical network proposed in the second document "M. S. Goodman, et al. "The LAMBDANET multiwavelength network: Architecture, applications and demonstrations," IEEE Journal on Selected Areas in Communications Vol. 8 No. 6 pp.995-1003, 1990".
In FIG. 14, each of terminals comprises one transmitter having a fixed wavelength and receivers having a fixed wavelength, the number of which corresponds to the number of the terminals, and the terminals are connected to each other through a star coupler and an optical fiber, thereby constituting a star optical network. The wavelengths of the transmitters at the respective terminals are independent of each other. Further, in the optical network, the synchronization of the terminals is established, whereby communication is established using TDM time slots. On the transmission side, a destination terminal to which information can be transmitted is periodically assigned for each TDM time slot. Further, the assignment of TDM time slots is determined on the transmission side in order that information are not simultaneously received from a plurality of terminals on the receiving side.
In the optical network, the terminals must be synchronized with each other, including delay time in order that transmission information from the plurality of terminals do not collide with each other on a transmission line on the receiving side. In order to establish the synchronization, however, a mechanism for preparing a new channel or the like and transmitting a reference clock signal, for example, is required, whereby the construction of the optical network itself becomes complicated. Further, filters and receivers are required for each wavelength, that is, for each terminal, whereby the scale of the terminals is also increased.
Furthermore, in the optical communication system, communication is established by the assignment of the TDM time slots. That is, a band is assigned to each of the terminals irrespective of the presence or absence of information to be sent, whereby the transmission efficiency of transmission data is reduced.
"Third Background Art"
FIG. 15 is a block diagram showing the construction of an optical network proposed in the third document "F. J. Janneillo, R. Ramaswami, and D. G. Steinberg, "A prototype Circuit-Switched Multiwavelength Optical Metropolitan Area Network", International Communication Conference pp818-823, 1992".
In FIG. 15, each of stations comprises one light transmitter having a fixed wavelength and one receiver comprising a variable wavelength filter, and the stations are connected to a star coupler through a fiber, thereby constituting a star optical network. Wavelengths assigned to the light transmitters in the stations are independent for each terminal.
In the optical network shown in FIG. 15, the station on the information transmission side continues to send out a transmission request signal when a transmission request is generated. The receiver in each of the stations periodically scans the wavelength passband of the variable wavelength filter, to examine whether or not a transmission request addressed to itself is sent out into the network. Each of the stations fixes the wavelength passband of the variable wavelength filter when the receiver receives a transmission request signal addressed to itself, to receive information subsequently sent from the station on the information transmission side.
In the construction of the optical network, the station having a transmission request continuously sends out a transmission request signal utilizing the light transmitter. The station on the receiving side operates the variable wavelength filter, to scan the wavelength, and examines whether or not a transmission request addressed to itself is sent out from the other station in a period during which it receives no data. The station on the receiving side starts communication at the time point where it detects that a transmission request from the other station is generated. In a period elapsed until communication is started, time required to propagate a signal on a transmission line and time required to scan the wavelength by the variable wavelength filter are necessarily required. That is, the synchronization of the stations need not be always established. However, the transmission efficiency of the information is not good, because information is not transmitted and received in a period elapsed from the time when the transmission request is generated until the destination responds to the transmittion request. The bus network has the possibility that the utilization efficiency of the transmission line is improved in simple construction because each of the terminals can recognize whether or not a signal from the previous terminals exists. Therefore, a proposition as described in the following fourth document has been made under the background that after a light amplifier is developed, the disadvantage with the above transmission loss can be overcome by using the light amplifier even in the bus network.
"Fourth Background Art"
FIG. 16 is a block diagram showing the construction of a U shaped bus optical network proposed in the fourth document "I. Chlamtac et al. "A Multibus Train Communication (AMTRAC) Architecture for High-Speed Fiber Optic Networks" IEEE Journal on Selected Areas in Communications Vol. 6 No. 6 pp.913-912, July 1988". In FIG. 16, each of terminals 1 to n comprises one wavelength variable transmitter and one receiver having a fixed wavelength. The terminals 1 to n send out, when they have information to be transmitted for each slot to a plurality of channels 1 to m upon establishment of slot synchronization, transmission information to a particular channel upon confirming that there is no transmission from the previous terminal.
In this system, communication is established for each slot, whereby slot synchronization must be established in the entire network. Consequently, the optical network has the same problem as the problem described in "Second Background Art". Further, the assignment of slots to be transmitted to the terminals is determined from the beginning, whereby the utilization efficiency of the transmission band is reduced.
"Fifth Background Art"
FIG. 17 is a block diagram showing the construction of "Optical Composite Transceiver" disclosed in U.S. Pat. No. 4,809,361. In FIG. 17, a transceiver (a connecting device) connects a plurality of terminals to an optical fiber cable, thereby constituting a U or S shaped bus optical network. In the bus optical network, a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) system is utilized as an access method. That is, each of the terminals transmits data while examining whether or not data to be transmitted by itself collides with data to be transmitted by the other terminal. Each of the terminals stops the transmission at the time point where it detects the collision, while continuing the transmission when it does not detect the collision. It is considered that the transmission of data has succeeded when each of the terminals does not detect the collision until the transmission of the data is terminated.
In the CSMA/CD system, correct synchronization is not required in the entire optical network. In order to detect whether or not transmission data at a certain terminal collides with a transmission signal from the other terminal on an optical fiber cable, the transmission packet length must not be less than a packet length corresponding to the reciprocating time on a transmission line constituting the bus optical network. That is, the transmission efficiency is reduced by attaching dummy information in order to satisfy the minimum packet length even when short information is sent. Further, the possibility that transmission collision occurs is increased when the number of terminals is increased, whereby the transmission efficiency is reduced as the number of terminals is increased.